The present invention relates to a screen printing method for printing cream solder, conductive paste or like paste on a substrate.
Conventionally, screen printing has been adopted for printing cream solder, conductive paste or like paste on substrates in a process of mounting electronic components on a substrate. This method comprises a process of setting a mask plate with pattern holes provided therein according to the printing portions and a process of printing paste on a substrate by squeezing through the pattern holes of the mask plate.
As a squeezing method for such screen printing, a method using a closed type squeegee head is known. In this method, a squeegee head provided with a paste storage container having an opening at the bottom thereof is used. With the opening abutted on the mask plate, the paste in the paste storage container is pressurized so that the paste is pushed and filled into the pattern holes of the mask plate through the opening. And, the squeegee head is slid on the mask plate, then the paste is filled into each pattern hole in order.
In such a conventional screen printing method using a closed type squeegee head, there are problems as described in the following. In the screen printing operation, the squeegee head is in a state such that the bottom opening thereof is always contacted on the surface of the mask plate. However, it is necessary to separate the squeegee head from the screen mask, for example, when making the arrangements for the purpose of type change and carrying out the maintenance of the apparatus.
In this case, the paste is always in contact with the mask plate via the bottom opening at the closed type squeegee head. Accordingly, when the squeegee head moves upward, some of the paste in the squeegee head is liable to remain sticking to the surface of the mask plate. And, in case there remains some paste on the mask plate, it is necessary to wipe off the paste sticking to the mask plate. Therefore, in the conventional method, there is a problem of such residual paste that is discarded wastefully. Moreover, it is necessary to wipe off each time the paste sticking to the mask plate, and consequently, there has been a problem of a loss of time required for the arrangements and maintenance.
On the other hand, cream solder or the like paste used for screen printing contains resin component. The resin component easily shows a hardening reaction. In order to prevent such hardening reaction of the paste, the paste is kept at a constant temperature in a refrigerator or the like until the paste is used. And, when the paste is used, it is timely taken out of the refrigerator, taking into account the predetermined time of leaving the paste in the atmosphere, and then the paste is placed into the squeegee head.
The viscosity of such paste will vary depending upon the ambient atmospheric conditions, and further, the viscosity is liable to change with the lapse of time. For example, (a) at the time when the paste is taken out of the refrigerator and placed in the printing apparatus, and (b) after starting to use the paste, when the paste has been left in the apparatus for a long time while the printing operation is discontinued, then suitable viscosity for printing cannot be obtained. Thus, when the viscosity of the past used for screen printing is not appropriate, it is impossible to obtain reliable printing qualities.
The present invention is intended to provide a screen printing method that may assure reliable printing qualities.
The screen printing method of the present invention comprises:
(A) a step of sliding a squeegee head with paste stored in the squeegee head on a mask plate while the paste is pressurized, and printing the paste on a substrate through a pattern hole of the mask plate;
(B) at least one step of (b) separating the squeegee head with the paste stored in the squeegee head from the mask plate; and
(e) pre-squeezing the squeegee head on the mask plate before the step (A).
The step of separating the squeegee head from the mask plate comprises:
(1) a step of discontinuing the pressure application to the paste stored in the squeegee head;
(2) a step of horizontally moving the squeegee head by a predetermined distance while the bottom end portion of the squeegee head is abutted on the mask plate in a state of the pressure application being discontinued; and
(3) a step of moving the squeegee head upward off from the mask plate.
The step of pre-squeezing comprises:
(1) a step of pressurizing the paste stored in the squeegee head in a state of the squeegee head being abutted on the mask plate; and
(2) a step of horizontally moving the squeegee head while the bottom end portion of the squeegee head is abutted on the mask plate in a state of the paste being pressurized.
With the configuration, it is possible to assure reliable printing qualities.
Further, by the screen printing method comprising the (b) step, it is able to reduce the quantity of paste discarded wastefully without being used for printing, in addition to the advantage described above. Also, it is possible to avoid carrying out extra work for arrangements and maintenance.
At the other time, the screen printing method comprising the (e) step, it is able to lower the viscosity of paste making it appropriate for screen printing, thereby further improving the printing qualities.